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Ubuntu 24.04 Bluetooth Serial Port Setup Guide

This guide explains how to configure Ubuntu 24.04 to act as a Bluetooth Serial Port (SPP) server to receive data from the ABCommander app.

1. Install Required Tools

sudo apt update
sudo apt install bluez bluez-tools

2. Enable Compatibility Mode (MANDATORY)

  1. Edit the service: sudo nano /lib/systemd/system/bluetooth.service
  2. Change ExecStart line to: ExecStart=/usr/libexec/bluetooth/bluetoothd --compat
  3. Reload: sudo systemctl daemon-reload && sudo systemctl restart bluetooth
  4. Permissions: sudo chmod 777 /var/run/sdp (repeat after restart).

3. Disable Conflicting Services

Ubuntu's ModemManager often "hijacks" serial ports, causing "Address already in use" errors.

sudo systemctl stop ModemManager

4. The "Sniffer" Script (ble_listen.sh)

Create a script with these contents for a clean connection:

#!/bin/bash
echo "--- Resetting Bluetooth Stack ---"
sudo pkill -9 rfcomm
sudo rfcomm release all
sudo hciconfig hci0 down
sudo hciconfig hci0 up
sudo sdptool add --channel=4 SP
echo "Waiting for connection on CHANNEL 4..."
sudo rfcomm listen 10 4  # Uses /dev/rfcomm10

5. Step-by-Step Operational Flow

Follow these steps exactly to ensure a successful connection without "Address already in use" errors:

Phase 1: Server Preparation (Ubuntu)

  1. Stop ModemManager: sudo systemctl stop ModemManager
  2. Run the Script: Execute ./ble_listen.sh in Terminal A.
    • It should say: Waiting for connection on channel 4.
    • Do NOT start the cat command yet.

Phase 2: App Connection (Android)

  1. Open App: Launch ABCommander on your phone.
  2. Click Connect: Select your Ubuntu laptop from the list.
  3. Verify Handshake:
    • Look at Terminal A (Ubuntu). It should change to: Connection from [MAC] to /dev/rfcomm10
    • The App status should change to: Status: Connected to... and the button should say Disconnect.

Phase 3: Data Sniffing

  1. Read Data: Open Terminal B (New Tab) and run: 
    sudo cat /dev/rfcomm10
  2. Action: Press buttons in the app. Commands like 1_ON will appear in Terminal B.

Phase 4: Clean Disconnect

  1. Stop Sniffer: In Terminal B, press Ctrl+C to stop cat.
  2. Disconnect App: Click Disconnect on the phone.
  3. Reset Server: In Terminal A, press Ctrl+C and run sudo rfcomm release all before the next test.

Troubleshooting

  • Address already in use: Run sudo rfcomm release all and ensure no cat or tail processes are running on /dev/rfcomm*.
  • Send failed: Usually means the physical connection is up but the TTY device (/dev/rfcomm10) failed to initialize. Restart the script.